Electrochemical cell

ABSTRACT

An electrochemical cell comprises as an anode, a lithium transition metal oxide or sulphide compound which has a [B 2 ]X 4   n−  spinel-type framework structure of an A[B 2 ]X 4  spinel wherein A and B are metal cations selected from Li, Ti, V, Mn, Fe and Co, X is oxygen or sulphur, and n− refers to the overall charge of the structural unit [B 2 ]X 4  of the framework structure. The transition metal cation in the fully discharged state has a mean oxidation state greater than +3 for Ti, +3 for V, +3,5 for Mn, +2 for Fe and +2 for Co. The cell includes as a cathode, a lithium metal oxide or sulphide compound. An electrically insulative lithium containing liquid or polymeric electronically conductive electrolyte is provided between the anode and the cathode.

[0001] THIS INVENTION relates to an electrochemical cell.

[0002] According to the invention, there is provided an electrochemicalcell, which comprises

[0003] as at least part of an anode, a lithium transition metal oxide orsulphide compound which has a [B₂]X₄ ^(n−) spinel-type frameworkstructure of an A[B₂]X₄ spinel wherein A and B are metal cationsselected from Li, Ti, V, Mn, Fe and Co, X is oxygen (O) or sulphur (S),and n− refers to the overall charge of the structural unit [B₂]X₄ of theframework structure, and the transition metal cation of which in itsfully discharged state has a mean oxidation state greater than +3 forTi, +3 for V, +3,5 for Mn, +2 for Fe and +2 for Co;

[0004] as at least part of a cathode, a lithium metal oxide or sulphidecompound; and

[0005] an electrically insulative lithium containing liquid or polymericelectronically conductive electrolyte between the anode and the cathode,such that, on discharging the cell, lithium ions are extracted from thespinel-type framework structure of the anode, with the oxidation stateof the metal ions of the anode thereby increasing, while a concomitantinsertion of lithium ions into the compound of the cathode takes place,with the oxidation state of the metal ions of the cathode decreasingcorrespondingly.

[0006] The compounds of the anode and cathode may, in particular, belithium metal oxide compounds.

[0007] While the cell can be a primary cell, it is envisaged that itmay, in particular, be a rechargeable or secondary cell in which thereverse reactions to those set out above, take place during charging ofthe cell.

[0008] Thus, spinel compounds have structures that can be represented bythe general formula A[B₂]X₄ given hereinbefore, and in which the X atomsare ideally arranged in a cubic-close-packed fashion to form anegatively charged anion array comprised of face-sharing andedge-sharing X tetrahedra and octahedra. In the formula A[B₂]X₄, the Acations and B cations occupy tetrahedral and octahedral sitesrespectively. In the ideal spinel structure, with the origin of the unitcell at the centre ({overscore (3)} m), the close-packed anions arelocated at the 32e positions of the space group Fd3m. Each unit cellcontains 64 tetrahedral interstices situated at threecrystallographically non-equivalent positions 8a, 8b and 48f, and 32octahedral interstices situated at the crystallographicallynon-equivalent positions 16c and 16d. In the A[B₂]X₄ spinel, the Acations reside in the 8a tetrahedral interstices and the B cations inthe 16d octahedral interstices. There are thus 56 empty tetrahedral and16 empty octahedral sites per cubic unit cell.

[0009] The framework structure of the lithium metal oxide compound ofthe anode thus has, as its basic structural unit, a unit of the formula[B₂]X₄ ^(n−) as hereinbefore described.

[0010] In the anode of the cell of the present invention, therefore, theB cations of the [B₂]X₄ ^(n−) host framework structure may be regardedas being located at the 16d octahedral positions, and the X anions asbeing located at the 32e positions of the spinel structure. Thetetrahedra defined by the 8a, 8b and 48f positions and the octahedradefined by the 16c positions of the spinel structure thus form theinterstitial space of the [B₂]X₄ ^(n−) framework structure for thediffusion of mobile Li⁺ cations.

[0011] The B cations of the framework structure may consist of onecationic type, or more than one cationic type of identical or mixedvalence to provide various [B₂]X₄ ^(n−) framework structures, theoverall charge of which can vary over a wide range.

[0012] Spinel compounds having the [B₂]X₄ ^(n−) framework structure mayalso be characterized by crystallographic space groups other than theprototypic cubic space group Fd3m, and may therefore not have the idealcubic-close-packed structures hereinbefore described. For example, inLi_(1+x)[Mn₂]O₄ compounds with 0<x<1, ie compounds in which A is Li, andB is Mn, the spinel structure is distorted, as a result of theJahn-Teller Mn³⁺ octahedral site ions, to tetragonal symmetry, and thecompound is characterized by the tetragonal space groups F4₁/ddm, or,alternatively, I4₁/amd in which the tetrahedral and octahedral sitenomenclature differs from that as defined by the space group Fd3m.

[0013] Furthermore, the anode need not necessarily be a stoichiometricspinel compound, but can instead be a defect spinel. Defect spinels arewell known in the large family of spinel compounds and can havevacancies on the A sites, or on the B sites, or on both the A sites andB sites. For example, compounds can be synthesized in which defects arecreated by varying the quantity of B cations in the framework structuresuch that additional Li⁺ cations can enter and leave the framework. Inthese instances additional Li⁺ cations can partially occupy the 16doctahedral sites normally occupied by the B-type cations. Under suchcircumstances these partially occupied octahedra can be considered toform part of the interstitial space. Conversely, compounds can also besynthesized, in which part of the interstitial space defined by the 8a,8b and 48f tetrahedral and 16c octahedral interstices of the spinelstructure can be occupied by B-type cations, thereby rendering theseparticular sites at least partially inaccessible to the mobile Lications. The [B₂]X₄ ^(n−) framework structure can contain in certaininstances a minor proportion, typically less than 10 atomic percent, ofcations other than the mobile Li-type cations, or the A and B-typecations, within the framework structure or within the interstitialspaces of the framework structure, and that could serve to stabilize thestructure. For example, doped spinels of stoichiometryLi_(1+δ)Mn_(2−δ)O₄ where 0<δ≦0,1, for example, Li_(1,03)Mn_(1,97)O₄ inwhich δ=0,03, and LiM_(δ/2)Mn_(2−δ)O₄ where M=Mg or Zn and 0<δ≦0,05, forexample, LiMg_(0,025)Mn_(1,95)O₄, are more stable to cycling than thestoichiometric spinel LiMn₂O₄.

[0014] The compound of the anode may be a stoichiometric spinel selectedfrom the group comprising Li₄Mn₅O₁₂, which can be written as(Li)_(8a)[Li_(0,33)Mn_(1,67)]_(16d)O₄ in ideal spinel notation;Li₄Ti₅O₁₂, which can be written as (Li)_(8a)[Li_(0,33)Ti_(1,67)]_(16d)O₄in ideal spinel notation; LiTi₂O₄ which can be written as(Li)_(8a)[Ti₂]_(16d)O₄ in ideal spinel notation; LiV₂O₄, which can bewritten as (Li)_(8a)[V₂]_(16d)O₄ in ideal spinel notation; and LiFe₅O₈,which can be written as (Fe)_(8a)[Fe_(1,5)Li_(0,5)]_(16d)O₄ in idealspinel notation.

[0015] Instead, the compound of the anode may be a defect spinelselected from the group comprising Li₂Mn₄O₉, which can be written as(Li_(0,89)□_(0,11))_(8a)[Mn_(1,78)□_(0,22)]_(16d)O₄ in spinel notation;and Li₂Ti₃O₇, which can be written as(Li_(0,85)□_(0,15))_(8a)[Ti_(1,71)Li_(0,29)]_(16d)O₄ in spinel notation.In defect spinels, the distribution of Li⁺ on the A and B sites can varyfrom compound to compound.

[0016] Instead, the compound of the anode may have a spinel-typestructure, which can be a stoichiometric or defect spinel, with amixture of transition metal cations such as a lithium-iron-titaniumoxide in which the lithium and iron cations are located on the A-sites,and lithium, iron and titanium cations on the B-sites.

[0017] In a preferred embodiment of the invention, the transition metalcations, Ti, V, Mn, Fe and Co, reside predominantly or completely on theB-sites of the spinel structure, while the Li cations residepredominantly or completely on the A-sites of the structure.

[0018] The lithium metal oxide compound of the cathode may also have aspinel-type framework structure. Thus, the framework structure of thelithium metal oxide compound of the cathode may then also have, as itsbasic structural unit, a unit of the formula [B₂]X₄ ^(n−) of an A[B₂]X₄spinel, as hereinbefore described, with the transition metal cations ofthe anode being more electropositive than those of the cathode.

[0019] In the compound of the cathode, A and B may be a metal cation ofone type, or a mixture of different metal cations. The compound of thecathode may be a stoichiometric or defect spinel compound, ashereinbefore described.

[0020] When the compound of the cathode has a spinel-type structure, itmay be selected from the group having as its B-type cations Li, Mn, Coor Ni, or mixtures thereof, such as Li_(x)Mn₂O₄ where 0<x≦1 andLi_(x)Co₂O₄ where 0<x−2, optionally doped with additional metal cationsto stabilize the structure as hereinbefore described.

[0021] Instead, the compound of the cathode may have another structuretype, for example a layered type structure such as that found within asystem defined by a formula Li_(x)Co_(1−y)Ni_(y)O₂ where 0≦y≦1 and0<x≦1.

[0022] In general, the anode compound will be selected from those spinelcompounds that offer a relatively low voltage vs pure lithium, typicallythose that offer 3V or less, while the cathode compound will be selectedfrom those spinel compounds that offer a relatively high voltage vs purelithium, typically those that offer between 4,5 V and 3 V. For example,a Li/Li_(4+x)Ti₅O₁₂ cell delivers on discharge at 100 μA/cm² (for 0<x<1)an average voltage of approximately 1,5 V, while a Li/Li_(x)Mn₂O₄ celldelivers on discharge at 100 μA/cm² (for 0<x<1) an average voltage ofapproximately 4 V. Therefore, a cell in accordance with the inventioncan have Li_(4+x)Ti₅O₁₂ as an anode and Li_(x)Mn₂O₄ as a cathode, andwill deliver approximately 2,5 V on discharge and which is approximatelytwice the voltage of a nickel-cadmium cell. In another example, aLi/Li₂Mn₄O₉ cell delivers a voltage of approximately 2,8 V over most ofthe discharge. Thus, a cell in accordance with the invention can have aLi_(2+x)Mn₄O₉ anode and Li_(x)Mn₂O₄ as cathode, and deliversapproximately 1,2 V on discharge, which is the typical voltage of anickel-cadmium cell. It is convenient to load such cells in a dischargedstate, ie with the following configurations:

Li₄Ti₅O₁₂/Electrolyte/LiMn₂O₄  (1)

Li₂Mn₄O₉/Electrolyte/LiMn₂O₄  (2)

[0023] Although it is convenient to load such cells in a dischargedstate, the cells may also be loaded in the charged state, if so desired.In this respect, the anodes of the invention have lithiated spinelstructures and delithiated spinel structures that have the [B₂]X₄ spinelframework as defined hereinbefore.

[0024] In (1), Li⁺ ions are extracted from Li[Mn₂]O₄ during charge witha concomitant oxidation of the manganese ions from an average valence of3,5 to higher values, and inserted into the Li₄Ti₅O₁₂ electrodestructure with a concomitant reduction of the titanium cations from theaverage valence state of +4 to lower values. During this process Li⁺ions are shuttled between the oxide structures without the formation ofany metallic lithium, the cell voltage being derived from changes in theoxidation state of the transition metal cations in the anode and cathodestructures.

[0025] The electrolyte may be a room temperature electrolyte such asLiClO₄, LiBF₄, or LiPF₆ dissolved in an appropriate organic salt such aspropylene carbonate, ethylene carbonate, dimethyl carbonate,dimethoxyethane, or appropriate mixtures thereof. Instead, however, itmay be any appropriate polymeric electrolyte such as polyethylene oxide(PEO)—LiClO₄, PEO—LiSO₃CF₃ and PEO—LiN(CF₃SO₂)₂, that operates at roomtemperature or at elevated temperature, eg at about 120° C.

[0026] The invention will now be described by way of non-limitingexamples, and with reference to the accompanying drawings in which:

[0027]FIG. 1 shows powder X-ray diffraction patterns of compoundssuitable for use as anode materials in rechargeable electrochemicalcells according to the invention;

[0028]FIG. 2 shows powder X-ray diffraction patterns of compoundssuitable for use as cathode materials in rechargeable electrochemicalcells according to the invention;

[0029]FIG. 3 shows a plot of voltage vs capacity for a known Li/Li₂Mn₄O₉cell;

[0030]FIG. 4 shows a plot of voltage vs capacity for a knownLi/Li₄Mn₅O₁₂ cell;

[0031]FIG. 5 shows a plot of voltage vs capacity for a knownLi/Li₄Ti₅O₁₂ cell;

[0032]FIG. 6 shows a plot of voltage vs capacity for a known Li/LiFe₅O₈cell;

[0033]FIG. 7 shows a plot of voltage vs capacity for a Li/Li-Fe-Ti oxidecell;

[0034]FIG. 8 shows a plot of voltage vs capacity for a known Li/LiMn₂O₄cell;

[0035]FIG. 9 shows a plot of voltage vs capacity for a knownLi/Li_(1,03)Mn_(1,97)O₄ cell;

[0036]FIG. 10 shows a plot of voltage vs capacity for a known Li/LiCoO₂cell;

[0037]FIG. 11 shows a plot of voltage vs capacity for the cell ofExample 1 and which is in accordance with the invention;

[0038]FIG. 12 shows a plot of voltage vs capacity for the cell ofExample 2 and which is in accordance with the invention;

[0039]FIG. 13 shows a plot of voltage vs capacity for the cell ofExample 3 and which is in accordance with the invention;

[0040]FIG. 14 shows a plot of voltage vs capacity for the cell ofExample 4 and which is in accordance with the invention;

[0041]FIG. 15 shows plots of voltage vs capacity for the cells ofExamples 5 and 6 and which are in accordance with the invention; and

[0042]FIG. 16 shows a cyclic voltammogram of the Li/Li-Fe-Ti oxidespinel cell of Example 7.

[0043] The following stoichiometric spinel and defect spinel compoundswere selected for use as anode materials in the examples followinghereinafter:

[0044] a) Li₂Mn₄O₉

[0045] b) Li₄Mn₅O₁₂

[0046] c) Li₄Ti₅O₁₂

[0047] d) LiFe₅O₈

[0048] e) Li-Fe-Ti oxide spinel in which Li:Fe:Ti=2:2:1

[0049] Powder X-ray diffraction patterns of these compounds are given inFIG. 1a-e respectively.

[0050] The following spinel and non-spinel compounds were selected foruse as cathode materials in the examples following hereinafter:

[0051] a) LiMn₂O₄ (spinel-type structure)

[0052] b) Li_(1,03)Mn_(1,97)O₄ (spinel-type structure)

[0053] c) LiCoO₂ (layered-type structure)

[0054] Powder X-ray diffraction patterns of these compounds are given inFIG. 2a-c respectively.

EXAMPLE 1

[0055] In view thereof that a Li/Li₂Mn₄O₉ cell delivers on discharge 150mAh/g at an average voltage of approximately 2,8 V, as indicated in FIG.3, and a Li/LiMn₂O₄ cell delivers on discharge 120 mAh/g at an averagevoltage of approximately 3,8 V, as indicated in FIG. 8, a cell inaccordance with the invention and having the configurationLi₂Mn₄O₉(anode)/Electrolyte/LiMn₂O₄(cathode) (2) was constructed.

[0056] The LiMn₂O₄ spinel compound of the cathode was synthesized byreaction of LiOH and γ-MnO₂ (chemically-prepared manganese dioxide, CMD)firstly at 450° C. for 48 hours and thereafter at 750° C. for 48 hours.The powder X-ray diffraction pattern of this compound is shown in FIG.2a.

[0057] Li₂Mn₄O₉ was synthesized by reaction of LiOH and MnCO₃ at 345° C.for 32 hours. The powder X-ray diffraction pattern of this compound isshown in FIG. 1a. The pattern is predominantly characteristic of theLi₂Mn₄O₉ defect spinel phase, but contains in addition a few very weakpeaks, for example at 42°2θ and 53°2θ, that are indicative of a veryminor proportion of lithiated γ-MnO₂ phase.

[0058] A cell of the format Li₂Mn₄O₉/Electrolyte/LiMn₂O₄ (2) was thenconstructed. The electrolyte used was 1M LiClO₄ in propylene carbonate.The first 9 charge and 8 discharge cycles of the cell are shown in FIG.11. A current of 0,1 mA was employed for both charge and discharge. Thecell was cycled between upper and lower voltage limits of 1,5 V and 0,45V respectively.

EXAMPLE 2

[0059] In view thereof that a Li/Li₄Mn₅O₁₂ cell delivers on discharge150 mAh/g at an average voltage of approximately 2,7 V, as indicated inFIG. 4, and a Li/Li_(1,03)Mn_(1,97)O₄ cell delivers on discharge 100mAh/g at an average voltage of approximately 3,9 V, as indicated in FIG.9, a cell in accordance with the invention and having the configurationLi₄Mn₅O₁₂/Electrolyte/Li_(1,03)Mn_(1,97)O₄ (3) was constructed.

[0060] The Li_(1,03)Mn_(1,97)O₄ spinel compound of the cathode wassynthesized by the reaction of LiOH and γ-MnO₂ (chemically-preparedmanganese dioxide, CMD) firstly at 450° C. for 48 hours and thereafterat 650° C. for 48 hours. The powder X-ray diffraction pattern of thiscompound is shown in FIG. 2b.

[0061] Li₄Mn₅O₁₂ was synthesized by the reaction of Li₂CO₃ and MnCO₃ at400° C. for 10 hours. The powder X-ray diffraction pattern of thiscompound is shown in FIG. 1b. The pattern is predominantlycharacteristic of the Li₄Mn₅O₁₂ spinel phase.

[0062] A cell of the format Li₄Mn₅O₁₂/Electrolyte/Li_(1,03)Mn_(1,97)O₄(3) was then constructed. The electrolyte used was 1M LiClO₄ inpropylene carbonate. The first 5 charge/discharge cycles of the cell areshown in FIG. 12. A current of 0,1 mA was employed for both charge anddischarge. The cell was cycled between upper and lower voltage limits of1,6 V and 0,5 V respectively.

EXAMPLE 3

[0063] In view thereof that a Li/Li₄Ti₅O₁₂ cell delivers on discharge120 mAh/g at an average voltage of approximately 1,5 V, as indicated inFIG. 5, and a Li/Li_(1,03)Mn_(1,97)O₄ cell delivers on discharge 100mAh/g at an average voltage of approximately 3,9 V, as indicated in FIG.9, a cell in accordance with the invention and having the configurationLi₄Ti₅O₁₂/Electrolyte/Li_(1,03)Mn_(1,97)O₄ (4) was constructed.

[0064] The Li_(1,03)Mn_(1,97)O₄ spinel compound of the cathode wassynthesized as in Example 2.

[0065] Li₄Ti₅O₁₂ was synthesized by the reaction of Li₂CO₃ and TiO₂,using a Li/Ti atomic ratio of 0,87, at 500° C. for 12 hours and at 1000°C. for 24 hours. A slight excess of lithium was used because of thevolatility of Li₂O at that temperature. The powder X-ray diffractionpattern of this compound is shown in FIG. 1c. The pattern ispredominantly characteristic of the Li₄Ti₅O₁₂ spinel phase.

[0066] A cell of the format Li₄Ti₅O₁₂/Electrolyte/Li_(1,03)Mn_(1,97)O₄(4) was then constructed. The electrolyte used was 1M LiClO₄ inpropylene carbonate. The first 7 charge/discharge cycles of the cell areshown in FIG. 13. A current of 0,1 mA was employed for both charge anddischarge. The cell was cycled between upper and lower voltage limits of2,8 V and 1,9 V respectively.

EXAMPLE 4

[0067] In view thereof that a Li/Li₄Ti₅O₁₂ cell delivers on discharge120 mA.Hrs/g at an average voltage of approximately 1,5 V, as indicatedin FIG. 5, and a Li/LiCoO₂ cell delivers on discharge 140 mA.Hrs/g at anaverage voltage of approximately 3,9 V, as indicated in FIG. 10, a cellin accordance with the invention and having the configurationLi₄Ti₅O₁₂/Electrolyte/LiCoO₂ (5) was constructed.

[0068] The LiCoO₂ spinel compound of the cathode was synthesized by thereaction of CoCO₃ and Li₂CO₃ firstly at 400° C. for 48 hours andthereafter at 900° C. for 48 hours. The powder X-ray diffraction patternof this compound is shown in FIG. 2c.

[0069] Li₄Ti₅O₁₂ synthesized as in Example 3, was used for the anode inthis example.

[0070] A cell of the format Li₄Ti₅O₁₂/Electrolyte/LiCoO₂ (5) was thenconstructed. The electrolyte used was 1M LiCoO₄ in propylene carbonate.The first 3 charge/discharge cycles of the cell are shown in FIG. 14. Acurrent of 0,1 mA was employed for both charge and discharge. The cellwas cycled between upper and lower voltage limits of 2,8 V and 1,9 Vrespectively.

EXAMPLE 5

[0071] In view thereof that a Li/LiFe₅O₈ cell delivers on discharge 100mAh/g at an average voltage of approximately 1,0 V, as indicated in FIG.6, and a Li/Li_(1,05)Mn_(1,97)O₄ cell delivers on discharge 100 mAh/g atan average voltage of approximately 3,9 V, as indicated in FIG. 9, acell in accordance with the invention and having the configurationLiFe₅O₈/Electrolyte/Li_(1,03)Mn_(1,97)O₄ (6) was constructed.

[0072] The Li_(1,03)Mn_(1,97)O₄ spinel compound of the cathode wassynthesized as in Example 2.

[0073] LiFe₅O₈ was synthesized by reacting of Li₂CO₃ and α-Fe₂O₃ in a1:5 molar ratio at 900° C. for 24 hours. The powder X-ray diffractionpattern of this compound is shown in FIG. 1d.

[0074] A cell of the format LiFe₅O₈/Electrolyte/Li_(1,03)Mn_(1,97)O₄ (6)was then constructed. The electrolyte used was 1M LiClO₄ in propylenecarbonate. The first charge cycle of the cell is shown in FIG. 15a. Acurrent of 0,1 mA was employed for both charge and discharge. The cellhad an upper voltage limit of 4,1 V.

EXAMPLE 6

[0075] In view thereof that a Li/Li-Fe-Ti oxide spinel cell delivers ondischarge 80 mAh/g at an average voltage of approximately 0,6 V, asindicated in FIG. 7, and a Li/Li_(1,03)Mn_(1,97)O₄ cell delivers ondischarge 100 mAh/g at an average voltage of approximately 3,9 V, asindicated in FIG. 9, a cell in accordance with the invention and havingthe configuration Li-Fe-Ti oxide spinel/Electrolyte/Li_(1,03)Mn_(1,97)O₄(7) was constructed.

[0076] The Li_(1,03)Mn_(1,97)O₄ spinel compound of the cathode wassynthesized as in Example 2.

[0077] A Li-Fe-Ti oxide spinel was synthesized by the reaction of Li₂CO₃and Fe₂TiO₅, using a Li:Fe:Ti atomic ratio of 2:2:1, at 500° C. for 6hours and at 900° C. for 16 hours. The powder X-ray diffraction patternof this compound is shown FIG. 1e. The pattern is predominantlycharacteristic of a spinel-type phase.

[0078] A cell of the format Li-Fe-Ti oxidespinel/Electrolyte/Li_(1,03)Mn_(1,97)O₄ (7) was then constructed. Theelectrolyte used was 1M LiClO₄ in propylene carbonate. The first chargecycle of the cell is shown in FIG. 15b. A current of 0,1 mA was employedfor both charge and discharge. The cell had an upper voltage limit of4,4 V.

EXAMPLE 7

[0079] A Li-Fe-Ti oxide spinel was synthesized by the reaction Li₂CO₃and Fe₂TiO₃ using a Li:Fe:Ti atomic ratio of 1:2:1 at 500° C. for 6hours, and thereafter at 900° C. for 16 hours. A cyclic voltammogram ofa Li/Li-Fe-Ti oxide spinel cell with an electrolyte of 1M LiCO₄ inpropylene carbonate is shown in FIG. 16. It shows the rechargeablecharacteristics of the Li-Fe-Ti oxide spinel electrode, and inparticular, the rechargeability of the Li insertion/extraction reactionthat occurs at approximately 1,5 V versus lithium.

[0080] Examples 5, 6 and 7 show, in particular, the potential of usingspinel-type oxides containing iron as anodes because they provide a lowvoltage against lithium. Furthermore, the experimental data provided inthe examples demonstrate the ability of transition metal oxides toprovide an electrochemical couple for ‘rocking chair’ rechargeablelithium cells in which lithium ions are transported between the twotransition metal oxide electrodes, the anode of which has a spinel-typestructure, and which uses a liquid or polymeric electrolyte containingLi⁺ ions. The electrochemical cells of the invention thus contain nometallic lithium anode, and are therefore inherently safer than lithiumcells containing metallic lithium anodes and, indeed, lithium-carbonanodes. In particular, such cells have an added advantage of providing amore constant operating voltage than cells with carbon anodes. Althoughthe cells of the invention are designed primarily for the use asrechargeable cells, they can also, as indicated hereinbefore, beutilized as primary cells, if desired.

[0081] Although the principles of this invention have been demonstratedby use of lithium-metal oxide compounds, the compounds of theelectrodes, instead of being oxides, can be sulphides.

1. An electrochemical cell, which comprises as at least part of ananode, a lithium transition metal oxide or sulphide compound which has a[B₂]X₄ ^(n−) spinel-type framework structure of an A[B₂]X₄ spinelwherein A and B are metal cations selected from Li, Ti, V, Mn, Fe andCo, X is oxygen (O) or sulphur (S) , and n− refers to the overall chargeof the structural unit [B₂]X₄ of the framework structure, and thetransition metal cation of which in its fully discharged state has amean oxidation state greater than +3 for Ti, +3 for V, +3,5 for Mn, +2for Fe and +2 for Co; as at least part of a cathode, a lithium metaloxide or sulphide compound; and an electrically insulative lithiumcontaining liquid or polymeric electronically conductive electrolytebetween the anode and the cathode, such that, on discharging the cell,lithium ions are extracted from the spinel-type framework structure ofthe anode, with the oxidation state of the metal ions of the anodethereby increasing, while a concomitant insertion of lithium ions intothe compound of the cathode takes place, with the oxidation state of themetal ions of the cathode decreasing correspondingly.
 2. A cellaccording to claim 1, wherein the compounds of the anode and the cathodeare lithium metal oxide compounds.
 3. A cell according to claim 1wherein, in the compound of the anode, B is a single transition metalcation type.
 4. A cell according to claim 1 wherein, in the compound ofthe anode, B is a mixture of different transition metal cations.
 5. Acell according to claim 1, wherein the compound of the anode is astoichiometric spinel selected from the group comprising Li₄Mn₅O₁₂,which can be written as (Li)_(8a)[Li_(0,33)Mn_(1,67)]_(16d)O₄ in idealspinel notation; Li₄Ti₅O₁₂, which can be written as(Li)_(8a)[Li_(0,33)Ti_(1,67)]_(16d)O₄ in ideal spinel notation; LiTi₂O₄which can be written as (Li)_(8a)[Ti₂]_(16d)O₄ in ideal spinel notation;LiV₂O₄, which can be written as (Li)_(8a)[V₂]_(16d)O₄ in ideal spinelnotation; and LiFe₅O₈, which can be written as(Fe)_(8a)[Fe_(1,5)Li_(0,5)]_(16d)O₄ in ideal spinel notation.
 6. A cellaccording to claim 1, wherein the compound of the anode is a defectspinel selected from the group comprising Li₂Mn₄O₉, which can be writtenas (Li_(0,89)□_(0,11))_(8a)[Mn_(1,78)□_(0,22)]_(16d)O₄ in spinelnotation; and Li₂Ti₃O₇, which can be written as(Li_(0,85)□_(0,15))_(8a)[Ti_(1,71)Li_(0,29)]_(16d)O₄ in spinel notation.7. A cell according to claim 1, wherein the compound of the anode is alithium-iron-titanium oxide having a spinel-type structure and in whichlithium and iron cations are located on the A-sites, and lithium, ironand titanium cations on the B-sites.
 8. A cell according to claim 1wherein, in the compound of the anode, the [B₂]X₄ framework structurecontains, within the framework structure or within the interstitialspaces of the framework structure, additional metal cations to thelithium ions and the A and B cations to stabilize the structure, withthe additional metal cations being present in an amount less than 10atomic percent.
 9. A cell according to claim 1, wherein the lithiummetal oxide compound of the cathode also has a spinel-type frameworkstructure.
 10. A cell according to claim 9, wherein the frameworkstructure of the lithium metal oxide compound of the cathode has as itsbasic structural unit, a unit of the formula [B₂]X₄ ^(n−), where [B₂]X₄^(n−) is the structural unit of an A[B₂]X₄ spinel, with the X anionsbeing arranged to form a negatively charged anion array, and wherein Ais a lithium cation; B is a metal cation; X is oxygen (O); and n− refersto the overall charge of the structural unit [B₂]X₄ of the frameworkstructure, with the transition metal cations of the anode being moreelectropositive than those of the cathode.
 11. A cell according to claim10 wherein, in the compound of the cathode, B is a single metal cationtype.
 12. A cell according to claim 10 wherein, in the compound of thecathode, B is a mixture of different metal cations.
 13. A cell accordingto claim 10, wherein the compound of the cathode is a spinel in whichthe B cation is selected from the group comprising Li, Mn, Co and Ni.14. A cell according to claim 10 wherein, in the compound of thecathode, the [B₂]X₄ framework structure contains, within the frameworkstructure or within the interstitial spaces of the framework structure,additional metal cations to the lithium ions and the A and B cations tostabilize the structure, with the additional metal cations being presentin an amount less than 10 atomic percent.
 15. A cell according to claim14, wherein the compound of the cathode is Li_(1+δ)Mn_(2−δ)O₄ where0<δ≦0,1.
 16. A cell according to claim 14, wherein the compound of thecathode is LiM_(δ/2)Mn_(2−δ)O₄ where M=Mg or Zn and 0<δ≦0,05.
 17. A cellaccording to claim 1, wherein the lithium metal oxide compound of thecathode has a layered-type structure conforming to the formulaLi_(x)Co_(1−y)Ni_(y)O₂ where 0<x≦1 and 0≦y≦1.
 18. A cell according toclaim 1, wherein the anode compound offers a relatively low voltage of 3V or less against pure lithium, while the cathode compound offers arelatively high voltage of between 3 V and 4,5 V against pure lithium.19. A cell according to claim 1, wherein the electrolyte is a roomtemperature electrolyte selected from the group comprising LiClO₄,LiBF₄, and LiPF₆ dissolved in an organic salt selected from the groupcomprising propylene carbonate, ethylene carbonate, dimethyl carbonate,dimethoxyethane and appropriate mixtures thereof.
 20. A cell accordingto claim 1, wherein the electrolyte is a polymeric electrolyte selectedfrom the group comprising polyethylene oxide (PEO)—LiClO₄, PEO—LiSO₃CF₃and PEO—LiN(CF₃SO₂)₂.